1. Field of the Invention
This invention relates to the shielding of electromagnetic and radio frequency waves, and more particularly to removable shielding apparatus for at least temporarily covering holes, apertures, or other openings in electronic equipment, such removable shielding apparatus being reusable.
2. Description of the Prior Art
Electronic equipment can emit a variety of electromagnetic waves that may cause interference with other unrelated equipment, such as radio, television or computer equipment. Electromagnetic interference (EMI), which includes radio frequency interference (RFI), can originate from computer equipment, or from associated power supplies, and can emanate from signal-transmitting wires and their connections. To contain EMI, electronic equipment manufacturers utilize cabinets made of electrically conductive materials arranged to totally encase all circuitry that can cause unwanted electromagnetic radiation. To comply with good design practice and to comply with governmental regulation, it is necessary to minimize electromagnetic radiation.
EMI considerations have become increasingly important partly because of the increased signal frequency operation of electronics devices. As the signal transition rate of electronic circuitry increases, so increases electromagnetic radiation. EMI shielding techniques are used to retain EMI within the operational device to thereby protect other equipment from interference generated in the shielded equipment, and also to protect the shielded equipment from external EMI sources. Shielding protects equipment from damage or improper operation (e.g., inadvertent triggering or switching of electronic circuits) due to current induction caused by the electromagnetic field. Governmental regulations and industry standards require that EMI and RFI sources within a system be shielded so that any unwanted EMI or RFI discharge will be maintained within regulation limits.
EMI gaskets, for sealing cabinet doom or other adjoining cabinet sections, and EMI covers, for covering cabinet openings, have been used across the path of EMI radiation to contain the electromagnetic emanation. These gaskets or covers are electrically conductive, and are designed to make electrical contact with the electronic equipment, or more particularly, to the equipment housing. The electromagnetic radiation that would normally pass through a hole or other aperture will be reflected back into the equipment housing to be dissipated in the resistive elements of the circuitry, and thus is prevented from escaping the enclosure.
Methods of coveting openings in electronic equipment have involved permanent affixation or adhesives. A permanent fix could include screwing or riveting a conductive cover over the opening. Such fixes may be effective in reducing EMI emanation, however it requires the drilling of more holes in the enclosure, which takes time, and the new screw or rivet holes can potentially become new sources of EMI leaks. A conductive cover could also be welded over the opening. Such covers are meant to be permanent, and are not readily removed. It may be the case that the opening in the enclosure is an intentional opening designed to allow for future expansion. For instance, an opening may be supplied in a computer cabinet to allow for cable connection to optional external peripheral devices. A permanent fix such as a riveted cover would be undesirable where there is a potential of using the opening at a future time.
Adhesives can be used to secure an EMI shield over an opening but adhesives can pose problems. First, an adhesive may exhibit the same undesirable effects as a permanently affixed cover if the adhesive can not be easily removed if so desired. Also, an adhesive can reduce the effectiveness of the electrical connection between the conductive material and the housing. To avoid this problem, conductive adhesives have been devised wherein a non-conductive material was made conductive by introducing conductive particles into the non-conductive material. Again, this method may result in a near-permanent cover, and the process of applying the conductive adhesive is time-consuming. Furthermore, conductive adhesives tend to break down over time.
The present invention avoids the aforementioned problems by providing an inexpensive, flexible, surface-conforming and removable shielding cover. This shielding cover is easily attached to a metal shielding cabinet through the use of a magnetic component that will cooperate with the cabinet and/or time structure to hold the cover in place. No drilling, riveting, screwing, or gluing is required. This attachment method allows the cover to be easily removed when the underlying opening is to be used. Such a cover has proved to be very beneficial in systems capable of future expansion, where openings are provided for switches, connectors, cables, or other components which are not used in a particular system configuration. The Magnetically-Attachable EMI Shielding Cover can be placed over these openings to attenuate EMI emanation, and when the opening is to be used, the cover can be easily removed, leaving no rivet holes, screw holes, or glue remnants.
The Magnetically-Attachable EMI Shielding Cover can be easily cut to a particular size for proper fitting over an opening, and also to allow the shielding cover to be fit into less accessible places than that of other shielding covers. The ability to cut the Magnetically-Attachable EMI Shielding Cover to size can also minimize manufacturing and packaging costs, since a standard size can be manufactured, and the user simply cuts out a cover of the desired size and shape.
Another advantage of the present invention is its low cost. The shielding cover is constructed of low cost materials which are easily obtainable. Actual construction of the cover is very inexpensive, as there are very few construction steps required.
Methods have been developed for attenuating EMI emanation from shielding enclosures. One such method is disclosed in U.S. Pat. No. 5,239,125 by Savage et al., issued on Aug. 24, 1993. Disclosed therein is an EMI shield for shielding junctions and cracks of EMI protected structures, which also maintains electrical continuity in comers, joints, and areas of anticipated cracking or delamination. The EMI shield disclosed by Savage et al. comprises a porous screen positioned over the crack, joint, comer, etc., upon which an overcoat of metal is deposited on and through the screen such that the screen is bonded to the underlying surface. Such a method appears to permanently bond the screen to the underlying surface. The advantage of the present invention over the Savage et al. design is the ability to easily move, or remove, the shield at any time, and particularly when the underlying aperture has a future potential use. The Savage et al. disclosure also appears to fortify seams and other discontinuities having a relatively small aperture, since the metal overcoat is deposited on and through the screen such that the screen is bonded "to the underlying surface". The present invention can reflect EMI radiation regardless of the size of the aperture.
U.S. Pat. No. 4,408,255 by Adkins, issued on Oct. 4, 1983, discloses a method of reducing the EMI effects generated by printed circuit boards by using magnetically permeable materials to absorb EMI radiation. The magnetic material is used in a different manner, and for a different purpose, than in the present invention. In Adkins, the magnetic material is used to absorb the EMI radiation, whereas the present invention uses the magnetic material as a securing component to hold an EMI-reflective component over an EMI-emanating aperture.
Another arrangement utilizing a magnetic attachment material is disclosed in U.S. Pat. No. 3,969,572 by Rostek, issued on Jul. 13, 1976. Rostek discloses an EMI gasket for use in sealing discontinuities in shielded cabinets, such as door openings. The elongated gasket is formed by spirally wrapping a conductive metal around juxtaposed strips of plastic foam and flexible plastic permanent magnets. The magnetic gasket holds one portion of the cabinet against another. The cabinet portions perform the shielding, and the gasket electrically seals the seams between the cabinet portions. This differs from the present invention where the material is an EMI cover, and not a gasket. The present invention performs the EMI shielding function, whereas the Rostek disclosure seals gaps between cabinet components that are coveting EMI radiation. The EMI shield of the present invention can be cut to a desired size, and can conform to the surface of the cabinet due to it's relative flexibility. This allows a cabinet opening to be immediately shielded, without the need for modifying the cabinet itself. The flexible and pliable characteristics of present invention allows cabinet openings to be covered where the openings have rough or burred edges, and even where the openings are on a non-planar surface of the cabinet. In Rostek, the EMI shielding device does not cover openings itself, but rather seals gaps created by juxtaposed cabinet components. Therefore, the Rostek device does not have the desired quality of immediate repair of openings without cabinet modification, or of shielding openings on non-planar surfaces. Furthermore, the Rostek disclosure teaches a gasket strip which is bonded to one portion of the cabinet, rather than being completely removable as in the present invention. This bonding appears to be accomplished through the use of a conductive adhesive. The EMI shield of the present invention requires no conductive adhesives, since the conductive component is held directly against the cabinet by using a magnetic attraction device.
In summary, the present invention avoids the problems encountered in many other types of EMI covers. Problems associated with adhesives, permanent affixation, installation time, and expense can be overcome through the use of the present invention. The device can be used as a permanent EMI-reduction device, or it can be used during system testing in the early stages of a product development cycle. It is particularly advantageous during the early development stages, since cabinet openings may exist due to undeveloped external devices, and it would be undesirable to permanently affix EMI covers over these openings when they will be used in later stages of the development cycle. For the foregoing reasons, the present invention can prove to be a very desirable EMI-reduction device.